Muslim ban explained: How Trump's plan will unfold

US President Donald Trump signed an executive order making major changes to America's policies on refugees and immigration.

He suspended the US refugee program for 120 days, banned all immigrants from seven Muslim countries for 90 days and ordered his administration to develop "extreme vetting" measures for immigrants from those countries to keep "radical Islamic terrorists" out of America.

The order, signed Friday, also bars all Syrians from entering the US, and gives preference in admission to Christians, whom he said are persecuted in majority Muslim countries.

Here is a look at what Trump ordered:

WHY DID TRUMP TARGET SEVEN COUNTRIES?

Three of them - Iran, Sudan and Syria - are on the State Department's list of state sponsors of terrorism.

The other four - Iraq, Libya, Somalia and Yemen - are designated "terrorist safe havens" by the State Department.

HOW IT AFFECTS SYRIANS

Trump's order directs the State Department to stop issuing visas to Syrian nationals and halts the processing of Syrian refugees.

President Donald Trump's immigration order sowed more chaos and outrage across the country Sunday, with travelers detained at airports, panicked families searching for relatives and protesters registering opposition to the sweeping measure that was blocked by several federal courts. AP Photo / Seth Wenig

That will remain in effect until Trump determines that enough security changes have been made to ensure that would-be terrorists can't exploit weaknesses in the current vetting system.

Syria is a concern because the Islamic State militant group that is behind terrorist acts around the world still operates in the country, which has been devastated by a nearly six-year-long civil war.

The suspension is intended to provide time to review how refugees are vetted before they are allowed to resettle in the United States.

Trump's order also cuts the number of refugees the United States plans to accept this budget year by more than half, to 50,000 people from around the world.

During the last budget year the US accepted 84,995 refugees, including 12,587 people from Syria. President Barack Obama had set the current refugee limit at 110,000.

The temporary halt to refugee admissions does include exceptions for people claiming religious persecution, so long as their religion is a minority faith in their country. That could apply to Christians from Muslim-majority countries.

CONFUSION OVER EXTREME VETTING

Trump's order did not spell out specifically what additional steps he wants to see the Homeland Security and State departments add to the country's vetting system for refugees.

Instead he directed officials to the review the refugee application and approval process to find any other security measures that can be added to prevent people who pose a threat from using the refugee program.

A woman snaps a cell phone photo as others hold up signs acknowledging the federal stay affecting immigrants trying to reach the United States issued by New York federal judge Ann M. DonnellyKathy Willens / AP

During the Obama administration, vetting for refugees included in-person interviews overseas, where they provided biographical details about themselves, including their families, friendships, social or political activities, employment, phone numbers, email accounts and more.

They also provided biometric information, including fingerprints. Syrians were subject to additional, classified controls that administration officials at the time declined to describe, and processing for that group routinely took years to complete.

CAN TRUMP LEGALLY DO THIS?

He may on security grounds.

A president has the power to shut down the refugee program completely without approval from Congress.

Federal law allows the president to bar entry to any immigrant "or any class" of immigrants if he deems them "detrimental to the interests of the United States," and to "impose on the entry of aliens any restrictions he may deem to be appropriate," according to the law:

U.S. Code 1182 - Inadmissable aliens: "Whenever the President finds that the entry of any aliens or of any class of aliens into the United States would be detrimental to the interests of the United States, he may by proclamation, and for such period as he shall deem necessary, suspend the entry of all aliens or any class of aliens as immigrants or nonimmigrants, or impose on the entry of aliens any restrictions he may deem to be appropriate."

CAN TRUMP BLOCK THOSE WITH VISAS IN THE US?

This will likely be decided by the courts.

Immigration officials denied entry to immigrants from the seven countries after Trump signed the executive order, prompting legal action by some immigrants.

A US federal judge has issued an emergency stay that temporarily blocks the government from sending people out of the country after they have landed at an American airport with valid visas.

The American Civil Liberties Union estimates the stay will affect 100 to 200 people detained at US airports or in transit, but government lawyers could not confirm that number.

Judge Ann Donnelly of the US District Court in New York made the order today.

TRUMP'S TERRORISM CONCERNS

Trump's executive order suspends all immigration from countries with terrorism concerns for 90 days.

The State Department said the three-month ban in the directive applied to Iraq, Syria, Iran, Sudan, Libya, Somalia and Yemen - all Muslim-majority nations.

The order also calls for Homeland Security and State department officials, along with the director of national intelligence, to review what information the government needs to fully vet would-be visitors and come up with a list of countries that don't provide it.

A picture of the note shared on Twitter by Hend Amry (@LibyaLiberty)

The order says the government will give countries 60 days to start providing the information or citizens from those countries will be barred from travelling to the United States.

The temporary ban extends to citizens of those seven countries who hold U.S. visas or green cards.

Anyone who was abroad when the executive order was signed is now barred from coming back to the country for at least three months.

There is an exemption for people whose entry into the country is deemed in the nation's interest, but it's unclear how that exemption may be applied.

Barring any travel to the U.S. from those seven countries, even temporarily, appears to at least partially fulfil a campaign promise Trump made to ban Muslims from coming to the United States until assurances can be made that visitors are properly vetted.

WHAT ABOUT OTHER IMMIGRATION?

The State Department now requires interviews for all visa applicants, except for those under 14, over 79 or who previously held a similar visa that expired less than 12 months prior.

Diplomats and other official applicants also are exempt. Previously, the interview requirement was waived for applicants applying within 48 months of expiration.

There are no changes to the Visa Waiver Program, under which citizens of 38 countries, most of them in Western Europe, including Australia, can visit the U.S. for 90 days or less without a visa.

As of 2015, however, visas are required for those who have visited any of the seven Muslim-majority nations of heightened terrorism concerns.